21-使用动态内存分配和友元的继承示例

13.7.3　使用动态内存分配和友元的继承示例

为演示这些有关继承和动态内存分配的概念，我们将刚才介绍过的baseDMA、lacksDMA和hasDMA类集成到一个示例中。程序清单13.14是这些类的头文件。除了前面介绍的内容外，这个头文件还包含一个友元函数，以说明派生类如何访问基类的友元。

程序清单13.14　dma.h

// dma.h -- inheritance and dynamic memory allocation
#ifndef DMA_H_
#define DMA_H_
#include <iostream>
// Base Class Using DMA
class baseDMA
{
private:
    char * label;
    int rating;
public:
    baseDMA(const char * l = "null", int r = 0);
    baseDMA(const baseDMA & rs);
    virtual ~baseDMA();
    baseDMA & operator=(const baseDMA & rs);
    friend std::ostream & operator<<(std::ostream & os,
                                     const baseDMA & rs);
};
// derived class without DMA
// no destructor needed
// uses implicit copy constructor
// uses implicit assignment operator
class lacksDMA :public baseDMA
{
private:
    enum { COL_LEN = 40};
    char color[COL_LEN];
public:
    lacksDMA(const char * c = "blank", const char * l = "null",
              int r = 0);
    lacksDMA(const char * c, const baseDMA & rs);
    friend std::ostream & operator<<(std::ostream & os,
                                     const lacksDMA & rs);
};
// derived class with DMA
class hasDMA :public baseDMA
{
private:
    char * style;
public:
    hasDMA(const char * s = "none", const char * l = "null",
              int r = 0);
    hasDMA(const char * s, const baseDMA & rs);
    hasDMA(const hasDMA & hs);
    ~hasDMA();
    hasDMA & operator=(const hasDMA & rs);
    friend std::ostream & operator<<(std::ostream & os,
                                     const hasDMA & rs);
};
#endif

程序清单13.15列出了类baseDMA、lackDMA和hasDMA的方法定义。

程序清单13.15　dma.cpp

// dma.cpp --dma class methods
#include "dma.h"
#include <cstring>
// baseDMA methods
baseDMA::baseDMA(const char * l, int r)
{
    label = new char[std::strlen(l) + 1];
    std::strcpy(label, l);
    rating = r;
}
baseDMA::baseDMA(const baseDMA & rs)
{
    label = new char[std::strlen(rs.label) + 1];
    std::strcpy(label, rs.label);
    rating = rs.rating;
}
baseDMA::~baseDMA()
{
    delete [] label;
}
baseDMA & baseDMA::operator=(const baseDMA & rs)
{
    if (this == &rs)
        return *this;
    delete [] label;
    label = new char[std::strlen(rs.label) + 1];
    std::strcpy(label, rs.label);
    rating = rs.rating;
    return *this;
}
std::ostream & operator<<(std::ostream & os, const baseDMA & rs)
{
    os << "Label: " << rs.label << std::endl;
    os << "Rating: " << rs.rating << std::endl;
    return os;
}
// lacksDMA methods
lacksDMA::lacksDMA(const char * c, const char * l, int r)
    : baseDMA(l, r)
{
    std::strncpy(color, c, 39);
    color[39] = ‘\0';
}
lacksDMA::lacksDMA(const char * c, const baseDMA & rs)
    : baseDMA(rs)
{
    std::strncpy(color, c, COL_LEN - 1);
    color[COL_LEN - 1] = ‘\0';
}
std::ostream & operator<<(std::ostream & os, const lacksDMA & ls)
{
    os << (const baseDMA &) ls;
    os << "Color: " << ls.color << std::endl;
    return os;
}
// hasDMA methods
hasDMA::hasDMA(const char * s, const char * l, int r)
        : baseDMA(l, r)
{
    style = new char[std::strlen(s) + 1];
    std::strcpy(style, s);
}
hasDMA::hasDMA(const char * s, const baseDMA & rs)
        : baseDMA(rs)
{
    style = new char[std::strlen(s) + 1];
    std::strcpy(style, s);
}
hasDMA::hasDMA(const hasDMA & hs)
        : baseDMA(hs) // invoke base class copy constructor
{
    style = new char[std::strlen(hs.style) + 1];
    std::strcpy(style, hs.style);
}
hasDMA::~hasDMA()
{
    delete [] style;
}
hasDMA & hasDMA::operator=(const hasDMA & hs)
{
    if (this == &hs)
        return *this;
    baseDMA::operator=(hs); // copy base portion
    delete [] style;         // prepare for new style
    style = new char[std::strlen(hs.style) + 1];
    std::strcpy(style, hs.style);
    return *this;
}
std::ostream & operator<<(std::ostream & os, const hasDMA & hs)
{
    os << (const baseDMA &) hs;
    os << "Style: " << hs.style << std::endl;
    return os;
}

在程序清单13.14和程序清单13.15中，需要注意的新特性是，派生类如何使用基类的友元。例如，请考虑下面这个hasDMa类的友元：

friend std::ostream & operator<<(std::ostream & os,
                                 const hasDMA & rs);

作为hasDMA类的友元，该函数能够访问style成员。然而，还存在一个问题：该函数如不是baseDMA类的友元，那它如何访问成员lable和rating呢？答案是使用baseDMA类的友元函数operator<<()。下一个问题是，因为友元不是成员函数，所以不能使用作用域解析运算符来指出要使用哪个函数。这个问题的解决方法是使用强制类型转换，以便匹配原型时能够选择正确的函数。因此，代码将参数const hasDMA &转换成类型为const baseDMA &的参数：

std::ostream & operator<<(std::ostream & os, const hasDMA & hs)
{
// type cast to match operator<<(ostream & , const baseDMA &)
   os << (const baseDMA &) hs;
   os << "Style: " << hs.style << endl;
   return os;
}

程序清单13.16是一个测试类baseDMA、lackDMA和hasDMA的小程序。

程序清单13.16　usedma.cpp

// usedma.cpp -- inheritance, friends, and DMA
// compile with dma.cpp
#include <iostream>
#include "dma.h"
int main()
{
    using std::cout;
    using std::endl;
    baseDMA shirt("Portabelly", 8);
    lacksDMA balloon("red", "Blimpo", 4);
    hasDMA map("Mercator", "Buffalo Keys", 5);
    cout << "Displaying baseDMA object:\n";
    cout << shirt << endl;
    cout << "Displaying lacksDMA object:\n";
    cout << balloon << endl;
    cout << "Displaying hasDMA object:\n";
    cout << map << endl;
    lacksDMA balloon2(balloon);
    cout << "Result of lacksDMA copy:\n";
    cout << balloon2 << endl;
    hasDMA map2;
    map2 = map;
    cout << "Result of hasDMA assignment:\n";
    cout << map2 << endl;
    return 0;
}

程序清单13.14～程序清单13.16组成的程序的输出如下：

Displaying baseDMA object:
Label: Portabelly
Rating: 8
Displaying lacksDMA object:
Label: Blimpo
Rating: 4
Color: red
Displaying hasDMA object:
Label: Buffalo Keys
Rating: 5
Style: Mercator
Result of lacksDMA copy:
Label: Blimpo
Rating: 4
Color: red
Result of hasDMA assignment:
Label: Buffalo Keys
Rating: 5
Style: Mercator